The water content is usually between three and ten percent, but can be as high as twenty percent. Opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. These color variations are a function of growth size into the red and infrared wavelengths. Opal is Australia's national gemstone.

The hardness (Mohs scale) : 5.5 - 6.5

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The gems traders often keep the loose opal gemstones with a cup of water nearby in the container so the stones will not be cracked. This kind of gemstone is easily cracked.